This invention relates to a digital system for controlling a motor, which moves a machine or other device accurately to a pre-selected new position. There are many uses for control systems of this type, but one of the primary reasons for development of the present invention relates to problems inherent in devices moved by lead screws.
Lead screw driven devices have a "backlash" problem if the position feedback information is derived from measurement of the lead screw rotation rather than measurement of the position of the element driven by the lead screw. If the movement of the device in the initial direction leads to an "overshoot", the corrective motion in the reverse direction has an unacceptable hysteresis, or margin of error, because of the backlash in the lead screw driving mechanism. Therefore, the aim in such a mechanism is to cause the device to "settle into" its final position by means of movement in one direction only, and to always move into its final position from the same direction.
Conventional analog servo systems usually employ a controller which outputs a control signal proportional to an error signal, representing the difference between the command input and the actual output. For optimal control of such systems, the output in response to a step function command will have a critically damped response. For reasonably accurate control an overshoot is required, and a velocity control measurement is usually needed for stability. There is a tendency of such systems to oscillate at zero, or null, position. For the reasons already discussed, that tendency creates a serious problem in a lead screw position control system.
The digital counterpart of the conventional analog servo system usually varies the frequency of the motor drive signal to vary the motion velocity. Broadly speaking, in the context of the present discussion, such a system does not function differently from the conventional analog servo system. It has the same susceptibility to difficulties caused by the backlash problem in lead screw position control systems.
One of the specific problems for which the present invention was developed was the control of paper cutting machines. The general practice with such machines heretofore has been to use a high speed motor, a low speed motor, and a brake. The strategy in such a system is to run at high speed until close to the target, then run at slow speed, and finally apply the brake to stop the machine instantaneously. Such a system is not very accurate, and the use of the brake is damaging to the control system. Furthermore, it requires measurement and braking of the driven element directly, because of the inertia of the driven element relative to the lead screw.
While the problem of hysteresis, or position inaccuracy due to "hunting", can be at least partially avoided by direct measurement of the location of the element whose position is being changed, this is often highly impractical, and excessively expensive. For one thing, the distance measurement must be continuous, and must relate to a specific starting point. In other words, continuous base point to end point measurement is required. If large distances are to be covered, as on a large machine table, a highly accurate, direct measurement of the moving element is extremely difficult. For this reason, it is very desirable to use lead screw measurement to determine position.
The problem of inaccuracy due to hunting also arises wherever there is "slack", or elasticity, in the driving system, if it is desired to measure position from the position of the driving mechanism. Such elasticity or slack exists in belt-driven systems, geared drives, fluid couplings, and the like.
Various efforts have been made to solve this problem by assuring that the device "settles in" to its new position from one direction only. One such approach is to use a stepper motor for final settling. There is an inherent cost problem in using a stepper motor, because of the gearing ratio required to provide the desired resolution accuracy. For example, if we assume a resolution of 0.001 inch is required, this necessitates 1,000 steps per inch, requiring a complex and costly control system and motor.
Another approach to the unidirectional settling problem has been the use of timed pulses directed to a motor to provide an incremental pulse train for the settling motion. This works quite well where friction and inertia are reasonably constant over the entire motion path, and the load is constant. But this rarely is the case.
The present invention provides a simple, but elegant, solution for the problems explained above. Furthermore, its accuracy, and relatively lower cost because of simplicity, make it a desirable position control system even where it would be feasible to provide direct measurement of the driven element. In other words, its usefulness is not limited to systems which involve the hysteresis problem, such as systems in which the lead screw is measured instead of the driven element.